The ITCZThe ITCZInternational Tropical International Tropical Convergence ZoneConvergence Zone

Air MassesAir Masses An air mass is a large volume of air with uniform

characteristics of temperature and humidity, acquired from its source region (where it comes from).

The characteristics of the air mass are determined by the climate of the place where it originated, for example if an air mass forms in a warm, dry place such as a desert, it will bring warm, dry weather conditions to the areas over which it passes.

Air masses which form over oceans are termed “MARITIME”.“MARITIME”. These will bring wet weather.

Air masses which form over land are called “CONTINENTAL”“CONTINENTAL” These will bring dry weather..

Air masses are also called POLAR, TROPICAL or ARCTICPOLAR, TROPICAL or ARCTIC depending on where they formed.

Therefore, an air mass originating over land in a tropical area is named TROPICAL CONTINENTAL (CT for short). It will bring warm dry weather conditions.

YOUR TURN:YOUR TURN:Decide what the following air masses will be Decide what the following air masses will be called. Write down your answers and beside called. Write down your answers and beside each describe the weather conditions it will each describe the weather conditions it will bring.bring.a) An air mass forming over an ocean in a tropical a) An air mass forming over an ocean in a tropical area.area.b) An air mass originating over land in a polar area.b) An air mass originating over land in a polar area.c) An air mass originating over an ocean in a polar c) An air mass originating over an ocean in a polar area.area.

Air masses in Africa and the ITCZAir masses in Africa and the ITCZ

The main air masses influencing the climate of The main air masses influencing the climate of

Africa are Africa are Tropical ContinentalTropical Continental and and Tropical Tropical

MaritimeMaritime. Their meeting place (the place . Their meeting place (the place

where they converge) is known as the where they converge) is known as the Inter Inter

Tropical Convergence Zone (ITCZ).Tropical Convergence Zone (ITCZ).

AFRICA

Tropical ContinentalTropical Continental

A warm, dry air mass given its origin over the tropical Sahara Desert. Brings warm, dry stable air. Extremely warm temperatures. No rainfall or cloud. Also brings a warm, dry Harmattan wind. Weather conditions are monotonous.

Tropical ContinentalTropical Continental

A warm, dry air mass given its origin over the tropical Sahara Desert. Brings warm, dry stable air. Extremely warm temperatures. No rainfall or cloud. Also brings a warm, dry Harmattan wind. Weather conditions are monotonous.

cT

Tropical MaritimeTropical Maritime

A warm, wet air mass given its origin over tropical oceans (the Atlantic). Brings moist, unstable air causing rainfall and thunderstorms. Temperatures can be hot and weather patterns vary.

Tropical MaritimeTropical Maritime

A warm, wet air mass given its origin over tropical oceans (the Atlantic). Brings moist, unstable air causing rainfall and thunderstorms. Temperatures can be hot and weather patterns vary.

mT

ITCZWhere the air masses converge

How does the ITCZ and its associated air masses How does the ITCZ and its associated air masses influence the climate of Africa?influence the climate of Africa?

The ITCZ shifts position throughout the year in relation to theThe ITCZ shifts position throughout the year in relation to the

apparent movement of the sun because of the way the earth apparent movement of the sun because of the way the earth

sits on its axis.sits on its axis.

In JulyIn July

The sun is directly overhead the Tropic of Cancer due to the The sun is directly overhead the Tropic of Cancer due to the

tilt of the earth. The ITCZ migrates northwards and is tilt of the earth. The ITCZ migrates northwards and is

positioned over the Tropic of Cancer.positioned over the Tropic of Cancer.

Tropic of Cancer

Equator

Tropic of Capricorn

23 ½ºN

0º

23½ºS

ITCZITCZ

ITCZ is ITCZ is positioned at the positioned at the Tropic of CancerTropic of Cancer

This means that Africa is dominated by the Tropical Maritime This means that Africa is dominated by the Tropical Maritime

air mass, bringing with it warm,wet and generally unstable air mass, bringing with it warm,wet and generally unstable

conditions. This explains why July is Africa’s wet season.conditions. This explains why July is Africa’s wet season.

ITCZ

cT

mT

Warm moist unstable Tropical Maritime Air dominates most of Africa.

ITCZ positioned at Tropic of Cancer

In JanuaryIn January

The sun is directly overhead the Tropic of Capricorn due to The sun is directly overhead the Tropic of Capricorn due to

the tilt of the earth. Therefore the ITCZ migrates south and is the tilt of the earth. Therefore the ITCZ migrates south and is

positioned over the Tropic of Capricorn.positioned over the Tropic of Capricorn.

23 ½ºN

0º

23½ºS

ITCZITCZ

ITCZ is positioned at ITCZ is positioned at the Tropic of Capricornthe Tropic of Capricorn

EquatorTropic of C

ancer

Tropic of Capricorn

This means that much of Africa is influenced by Tropical This means that much of Africa is influenced by Tropical

Continental air, bringing with it warm, dry stable conditions Continental air, bringing with it warm, dry stable conditions

and the warm, drying Harmattan wind. This explains why and the warm, drying Harmattan wind. This explains why

most of Africa experiences its dry season at this time of year.most of Africa experiences its dry season at this time of year.

ITCZ cT

mT

Warm, dry stable Tropical Continental Air dominates most of Africa.

ITCZ positioned at Tropic of Cancer

Some western and southern parts of Africa remain under the Some western and southern parts of Africa remain under the

influence of Tropical Maritime air because the Tropical influence of Tropical Maritime air because the Tropical

Continental air mass (above the ITCZ) is not strong enough to Continental air mass (above the ITCZ) is not strong enough to

push against the Tropical Maritime air mass which is moving push against the Tropical Maritime air mass which is moving

into the land from the sea.into the land from the sea.

In March and September the sun sits directly overhead the In March and September the sun sits directly overhead the

Equator, so at these times of year the ITCZ is positioned over Equator, so at these times of year the ITCZ is positioned over

the Equator.the Equator.

ITCZ JULYITCZ

JANUARY

Link to ITCZ animation

Problems with the ITCZ in recent yearsProblems with the ITCZ in recent years

In recent years the ITCZ has not migrated as far north as it In recent years the ITCZ has not migrated as far north as it should do in July. It has only shifted position to 14should do in July. It has only shifted position to 14ºN of the ºN of the Equator. This means that the Tropical Maritime (warm, Equator. This means that the Tropical Maritime (warm, wet) air does not reach the Sahel region of Africa. wet) air does not reach the Sahel region of Africa. Instead, this part of Africa is dominated by Tropical Instead, this part of Africa is dominated by Tropical Continental air, resulting in hot dry conditions and a Continental air, resulting in hot dry conditions and a summer drought.summer drought.

In Winter (January) the ITCZ migrates to its position at the In Winter (January) the ITCZ migrates to its position at the Tropic of Capricorn so the Sahel region remains dominated Tropic of Capricorn so the Sahel region remains dominated by the warm, dry Tropical Continental Air. This means that by the warm, dry Tropical Continental Air. This means that for most of the year the Sahel region of Africa is influenced for most of the year the Sahel region of Africa is influenced by extreme heat and dry conditions, causing drought, soil by extreme heat and dry conditions, causing drought, soil erosion, desertification and land degradation.erosion, desertification and land degradation.

Read the handout about the problems with the ITCZ that your teacher has given you then file it into your folder.

Read the handout about the problems with the ITCZ that your teacher has given you then file it into your folder.

Task Time!Task Time! Use your notes on the ITCZ to answer the Use your notes on the ITCZ to answer the

following questions:following questions:

1) Write a definition for the ITCZ.2) Why does the ITCZ appear to migrate to different positions

throughout the year?3) Where is the ITCZ positioned in a) July b) January c) September?4) Explain why most of Africa experiences warm, wet weather in July.5) Explain why most of Africa experiences its dry season in January.6) Why do some parts of western and southern Africa always

experience Tropical Maritime air?7) What has happened to the movement of the ITCZ in recent years?8) Describe the consequences of this movement on the Sahel region of

Africa.9) Have a go at ACTIVITY 6 on page 7 of the Higher Geography

textbook

Example exam question (1)Example exam question (1)From Paper 1 2007:From Paper 1 2007:Study reference diagrams Q1A and Q1B. Identify air masses A and B, Study reference diagrams Q1A and Q1B. Identify air masses A and B,

and describe their origin and nature. (6)and describe their origin and nature. (6)

How to answerHow to answerAir mass A is Tropical Continental or cT. It originates over the

Sahara Desert in tropical latitudes, hence its name, tropical

continental. It brings warm, dry stable air and extremely warm

temperatures to the areas over which it passes. There is a

lack of rainfall or cloud.

Air mass B is Tropical Maritime or Mt. It originates over the

Atlantic ocean in tropical latitudes. It brings warm, moist

unstable air, causing rainfall and thunderstorms.

Example exam question (2)Example exam question (2)Describe and explain the varying rainfall patterns shown in referenceDescribe and explain the varying rainfall patterns shown in reference

diagram Q1B. (12)diagram Q1B. (12)

How to answerHow to answerTimbukto has on average very little rainfall (only 250mm per annum). It has no rainfall at all in January, February November and December. Jos too has distinct wet and dry seasons, experiencing 1000mm of rainfall per annum, a peak of 300mm in July and minimal rainfall in November, December, January and February. In contrast, Lagos has markedly more rainfall throughout the year, with up to 2000mm per annum, and rainfall peaking twice in the year. (June at 475mm and October at 200mm). It also has no dry months at all.

These patterns can be attributed to the movement of the ITCZ and its associated air masses. Lagos is under the influence of warm moist Tropical Maritime air for most of the year, which explains its higher annual rainfall amounts and lack of dry season. Its twin rainfall peaks are caused by the ITCZ migrating northwards and southwards in relation to the position of the sun. Timbukto and Jos in contrast aremuch further North than the ITCZ in January, which is positioned at the Tropic of Capricorn. This will result in Timbukto and Jos being influenced by warm, dry Tropical Continental air, which explains why they have either No or very little rainfall during these months.

In June and July Timbukto and experiences its highest rainfall amounts because the ITCZ has migrated northwards due to the position of the sun overhead the Tropic of Cancer. This results in Timbukto experiencing Tropical Maritime air at these times, hence its this is when it gets the majority of rainfall. Similarly, this is why Jos experiences its highest rainfall totals (300mm and 275mm respectively) in July and August.

1) Collect a copy of the TWO past paper questions.2) Read each question carefully, then write a detailed answer to each.3) Get your answers checked by your teacher.

NOW YOU HAVE A GO – REMEMBER DON’T PANIC, THIS IS DIFFICULT!!!